Submicron beta silicon carbide powder and sintered articles of high density prepared therefrom

ABSTRACT

Shaped silicon carbide ceramic articles of high density, e.g., at least 90 percent of theoretical, are produced by cold pressing and sintering boron-containing high purity, submicron beta silicon carbide powder. The silicon carbide powder is produced preferably by gas phase reaction of silicon halide, e.g., silicon tetrachloride, carbon source reactant, e.g., halogenated hydrocarbon, and boron source reactant, e.g., boron trichloride, with a hydrogen plasma.

This is a continuation of application Ser. No. 637,342, filed Dec. 3,1975, now abandoned.

DESCRIPTION OF THE INVENTION

For many years, it has been known that silicon carbide has outstandingmechanical and physical properties; but, it has also been appreciatedthat these properties are realized only in a continuous silicon carbidematrix with little or no porosity. Silicon carbide powder is not easilyself-consolidated to a density approaching theoretical. While highdensities can be achieved by the simultaneous application of very highpressures and temperatures, e.g., pressures greater than 20 kilobars andtemperatures above 2000° C., the limitation of such a process torelatively simple shapes and the high cost inherent in such a formingprocess and any required subsequent machining step has been a deterrentto utilization of silicon carbide as a high temperature ceramic.

It is known that the pressure and temperature required for hot pressingsilicon carbide can be reduced with the aid of small additions ofelements such as aluminum, iron and boron, or compounds thereof, to thesilicon carbide. See for example the articles, "Pressure-SinteredSilicon Carbide" by R. A. Alliegro et al, J. Amer. Ceram. Soc. 39 (11),386-389 (1956), and "The Preparation and Some Properties of Materials onthe Base of Silicon Carbide with Boron and Aluminum Additions" by N. D.Antonova et al., Sov. Pow. Met. and Met. Ceram., No. 6, pp. 444-449(1962). U.S. patents that describe silicon carbide-boron carbidecompositions include U.S. Pat. Nos. 2,108,794, 2,329,085, 2,964,823 and3,520,656. For example, U.S. Pat. No. 2,108,794 recites, "The additionof small percentages of boron carbide to silicon carbide makes possiblethe fusion of the material under conditions where silicon carbide itselfcannot be fused" (Column 6, lines 19-22). A lightning arrester valve ofsilicon carbide and one percent boron carbide is described in U.S. Pat.No. 2,329,085.

The literature describes a variety of methods for preparing siliconcarbide. A widely reported commercial process for preparing siliconcarbide is the reaction of sand and coke in a high temperature furnace.This process produces principally the alpha or hexagonal crystal formsof silicon carbide and yields a reaction mass that must be milledextensively and then screened to obtain a granular powder. Preparationof silicon carbide by reaction of a vaporizable silicon compound and ahydrocarbon in the presence of hydrogen in the vapor phase has beendescribed in U.S. Pat. Nos. 2,952,598, 3,340,020, 3,399,980, 3,508,954and 3,755,541. It is reported that the vapor phase reaction yieldsprincipally the beta or cubic crystal form of silicon carbide. However,U.S. Pat. No. 3,340,020 describes the preparation of alpha siliconcarbide by this method. Finally, U.S. Pat. No. 3,271,109 describes thepreparation of pigmentary silicon carbide of the beta crystal form bythe reaction of silicon oxide and carbon.

It has now been discovered that boron-containing submicron, beta siliconcarbide powders which are especially useful, notably because of theirability to be cold pressed and sintered into silicon carbide articleshaving high densities, e.g., at least 90 percent of theoretical, can beproduced by reacting a vaporizable compound of silicon, i.e., a silanesuch as silicon tetrachloride, a volatile source of boron, e.g., a boronhalide, and a volatile source of carbon, e.g., a halogenatedhydrocarbon, in a hot hydrogen stream produced by heating hydrogen in aplasma generator. The hot hydrogen stream is often referred to as ahydrogen plasma. Substantially all, i.e., at least 90 percent of theparticles that make up the powder have an equivalent spherical diameterless than one micron. The preponderant number, i.e., greater than 50percent, of the particles less than one micron are in the particle sizerange of between 0.05 and 0.7 microns. Moreover, X-ray diffractionanalysis of the silicon carbide powder indicates that it issubstantially all of the beta (cubic) crystal form. Only traces of alphasilicon carbide can be detected in the powder. Based on the sensitivityof the measuring instrument, it is believed that less than 1 weightpercent of the powder is of the alpha crystal form.

Submicron, beta silicon carbide powder can also be produced containingiron or aluminum as the densification aid (dopant) by introducing intothe hot hydrogen stream a suitable vaporizable compound of suchelements, e.g., a halide thereof, along with the volatile source ofsilicon and carbon. Boron is preferred as the densification aid. Boronis not only an excellent dopant; but, is most conveniently employedbecause its halide, e.g., the trichloride, is more volatile than thecorresponding iron or aluminum compounds. When silicon carbide isproduced in this manner, the carbide of the dopant's cation (for exampleboron carbide) is believed to be coproduced or coformed in submicronsize and homogeneously distributed throughout the silicon carbideproduct. Consequently, a truly homogeneous blend of a multicomponentsystem is produced. By virtue of such homogeneous distribution, moreefficient use of the dopant results. Consequently, the level of dopantrequired to achieve a desired level of densification is less when thedopant is coformed than when the dopant is physically blended withpre-formed silicon carbide.

The boron-containing submicron, beta silicon carbide powder produced bythe process described herein is unique as evidenced by its ability to beeffectively processed by cold forming and sintering techniques. It hasbeen found that this powder can be formed into simple and complexsilicon carbide shapes possessing densities approaching theoretical byconventional cold pressing and sintering, i.e., pressureless sintering,techniques. Thus, through the use of silicon carbide of the typeproduced by the process herein described it is possible to prepareshaped ceramic silicon carbide articles of high density, e.g., at least90 percent of the theoretical density of siliconcarbide, by using themore conventional ceramic cold forming techniques followed by sinteringsuch shaped articles under vacuum or atmospheric pressure in aconventional high temperature furnace. Further, a relatively smallamount of boron dopant is required to achieve such high densities withthe above-described silicon powder carbide. As little as 0.17 weightpercent boron has been found to promote densification of silicon carbideto levels of at least about 90 percent of theoretical. Moreover, aslittle as 0.24 weight percent boron has been found to promotedensification to levels of at least about 98 percent of theoretical.

The boron-containing, submicron beta silicon carbide powder prepared inaccordance with the process described herein can be cold pressed andsintered to produce dense ceramic articles which retain substantiallythe beta crystal form. This characteristic of the above-describedsilicon carbide powder is beneficial for the reason that alpha crystalsin the sintered article are considered strength limiting. Consequently,a dense article, the microstructure of which is of the beta crystal formhas the potential for achieving higher strength than an article in whichthe microstructure is of the alpha form. Further, the grain size of suchsintered articles is fine grained and uniform and not coarse. This isbelieved to be a result of the submicron size of the starting powder,which is capable of being sintered at a temperature low enough so thatcrystal growth is quite moderate.

The uniqueness of the submicron beta silicon carbide powder describedherein is believed to be attributable to one or more of the propertiesdescribed hereinafter. The silicon carbide particles that comprise thepowder product are substantially all submicron in size. Very few of theparticles are greater than 1 micron. Moreover, a large percentage of theparticles are within a fairly narrow size range, e.g., between 0.05 and0.7 microns. Further, the silicon carbide crystals are well formed withwell developed faces.

The aforesaid silicon carbide powder is of high purity. Because of themanner in which it is prepared, oxygen contamination is typically below0.5 percent, more usually less than 0.4 percent. Since oxygen isreported to inhibit densification, control of the oxygen level in thesilicon carbide is essential. The process for preparing the siliconcarbide powder described herein is especially advantageous because itpermits controlling oxygen contamination at very low levels.

The level of free carbon (chemically uncombined carbon) in the siliconcarbide powder can also be controlled at low levels because of theprocess used to prepare the powder. Thus, free carbon levels of 2 weightpercent or less, e.g., 1.5 or 1.0 weight percent or less, are capable ofbeing achieved. A small amount of free carbon in the silicon carbidepowder is reported to be beneficial since it may act as a scavenger foroxygen or elemental silicon that may be present in the powder. Theamount of other metallic elements (other than boron, which isintentionally added) present in the silicon carbide powder is also low.Commonly, less than 1000, e.g., 800 parts of other metallic elements permillion parts of silicon carbide powder, are present in the siliconcarbide powder. Such elements include: iron, aluminum, tungsten, copper,molybdenum, calcium, magnesium, titanium, manganese and vanadium.

The aforesaid silicon carbide powder is substantially all of the beta(cubic) crystal form. It is reported that the beta to alphatransformation is promoted by the presence of the alpha crystal forms.Consequently, the above-described beta silicon carbide powder is capableof being sintered to silicon carbide shaped articles havingpredominantly the beta crystal form.

The boron-containing compound that serves as the densification aid ishomogeneously dispersed throughout the silicon carbide powder with whichit is coproduced. As indicated, the boron-containing compound promotesdensification at relatively low temperatures, e.g., 1850°-2150° C.thereby aiding the preparation of a dense ceramic article having thebeta crystal form-high temperatures favoring the transformation of thebeta to alpha crystal forms.

Finally, it has been found that non-doped submicron beta silicon carbidepowder can be densified to at least 90 percent of its theoreticaldensity by blending with it coproduced boron-containing submicron betasilicon carbide in amounts such that the amount of boron present in themixture is at least about 0.20 percent. For example, about 74 parts ofsubmicron beta silicon carbide containing about 0.27 percent coproducedboron was blended thoroughly with about 26 parts of submicron betasilicon carbide substantially free of boron (<0.01%). This powder blend(the boron content of which was about 0.20 percent) was isostaticallypressed and sintered to a density of about 96 percent of theoretical.

DETAILED DESCRIPTION

The present invention relates to submicron beta silicon carbide powdercontaining minor densifying amounts of coformed boron-containingadditive, e.g., boron carbide, the process for producing such powder,and consolidated dense articles prepared from such silicon carbidepowder. The amount of boron-containing additive utilized in thesubmicron beta silicon carbide powder described herein is a densifyingamount, i.e., sufficient to obtain a densities by cold pressing andsintering techniques of at least 85, preferably at least 90 and, morepreferably at least 95 percent of theoretical. Typically, the amount ofboron-containing additive, calculated as percent elemental boron, variesbetween about 0.15 and about 4 weight percent. As used herein, the term"boron-containing additive", unless otherwise defined, is intended tomean boron present as elemental boron, chemically combined boron, e.g.,boron carbide, or both. It is believed from the evidence at hand thatwith as little as 0.15 weight percent of boron densities of at leastabout 85 percent of the theoretical density of silicon carbide, i.e.,3.21 grams/cc., may be obtained by cold pressing and sinteringtechniques. At boron levels of between about 0.15 and about 0.20 weightpercent, e.g., 0.17 weight percent, densities of at least 90 percent oftheoretical can be obtained; and, with from about 0.20 to about 0.25percent boron, densities of at least 95, e.g., 98 percent of theoreticalare obtained by such forming techniques. Boron concentrations above 0.25weight percent, e.g., between 0.3 and 3 or 4 weight percent can be used.A small amount of the boron-containing additive, e.g., up to 0.2-0.3weight percent, appears by X-ray diffraction analyses to be in solidsolution with the silicon carbide as evidenced by the absence of aseparate phase. Boron concentrations above about 4 weight percent do notappear to offer any advantages with respect to densification to the useof lower concentrations and may be detrimental to the properties of thedensified article. Consequently, the use of relatively lowconcentrations, from about 0.15 or 0.2 to about 1 or 2 weight percent,e.g., 0.15 or 0.2 to about 0.5 or 0.75 weight percent of boron, arepreferred.

The submicron beta silicon carbide powder compositions of the presentinvention can be prepared by blending homogeneously coformedboron-containing submicron beta silicon carbide and submicron betasilicon carbide powder that is essentially boron-free. Theboron-containing silicon carbide is used in an amount sufficient toprovide in the total blend an amount of boron equal to the valuesdescribed hereinbefore, i.e., in an amount equivalent to from about 0.15to about 4 weight percent boron. Alternatively and preferably theboron-containing submicron beta silicon carbide powder is prepared byproducing the boron-containing additive in situ during production of thesilicon carbide powder. Various advantages accrue when the boronadditive is introduced into the silicon carbide powder at that time.First, a more homogeneous distribution of boron in the silicon carbidepowder product results than can be achieved by physically blending aboron additive with preformed silicon carbide. Second, it eliminatespossible contamination of the silicon carbide with impurities that maybe introduced during a blending procedure, and thirdly it avoids thepossibility of producing non-homogeneous blends. Silicon carbide powdercontaining coproduced boron additive, i.e., boron additive formedsimultaneously in the reactor with the silicon carbide, providescompositions in which the efficiency of the boron as a densifying aid ismaximized. Consequently, less boron-containing additive is required toproduce the same results as when a physical blend of, for example,silicon carbide and boron or boron carbide is used. It is postulatedthat the essentially homogeneous dispersion of reactor added boron andits submicron particle size is a major reason for this result.

As indicated, a preferred method for incorporating boron-containingadditive in submicron beta silicon carbide powder is by coproducing theadditive simultaneously with the silicon carbide powder. This isaccomplished conveniently by introducing boron source reactant into thereactor wherein the silicon carbide powder is formed in amountssufficient to obtain a boron concentration in the final powder productas described hereinbefore. Because of the nature of the process(described hereinafter) used for the preparation of submicron betasilicon carbide powder, it is believed that the boron-containingadditive is present probably in the submicron silicon carbide assubmicron boron carbide, e.g, B₄. Further, because boron carbide issoluble to a limited extent in silicon carbide, a portion of the boronor boron carbide is probably in solid solution with the silicon carbide,i.e., to the limits of its solubility.

Among the boron source reactants that can be introduced into the reactorin which the silicon carbide is formed, there can be mentioned inorganicboron compounds such as boron tribromide, boron triiodide, borontrichloride, boron trifluoride, and the hydroborides (boranes), e.g., B₂H₆, B₅ H₉, B₁₀ H₁₄, and B₆ H₂. Boron trichloride, BCl₃, is preferred.The boron source reactant should be substantially oxygen-free andsubstantially pure to avoid the introduction of oxygen and metalimpurities into the silicon carbide powder product. By oxygen-free ismeant that the boron source reactant is substantially free of chemicallycombined oxygen, e.g., the oxides of boron. The boron source reactant isintroduced into the reactor in such a manner that it is present in thereaction zone as a vapor. It is chosen from those enumerated compoundsof boron that react in a thermodynamically favorable manner with thesilane and carbon source reactants at the chosen reaction temperaturefor the production of slicon carbide.

Generally, any readily volatile inorganic or organic silane can be usedas the silicon source reactant in the preparation of submicron betasilicon carbide powder by the process described herein. As used herein,the term "silane" is intended to mean and include compounds containingsilicon and elements selected from the group, hydrogen, halogen, i.e.,chlorine, bromine, iodine, and fluorine, hydrogen and halogen andhydrogen, carbon and halogen. Silanes that can be used as the reactantinclude: the tetrahalosilanes, such as silicon tetrachloride,tetrabromide, and tetraiodide; the hydrosilanes (hydrosilicides), e.g.,SiH₄, Si₂ H₆, Si₃ H₈, etc. halogenated hydrosilanes, e.g., SiH₃ Cl, SiH₂Cl₂, and SiHCl₃ ; and haloalkyl silanes, such as trihaloalkyl silanes,e.g., trichloromethyl silane. The tetrahalosilanes, such as silicontetrachloride, are preferred. In the preparation of submicron siliconcarbide containing coformed boron additive, it is preferred that thehalogen of the silane is the same as the halogen of the boron sourcereactant, e.g., silicon tetrachloride and boron trichloride. The silanesalso should be substantially pure, i.e. substantially free of metalcontaminants and substantially free of chemically combined oxygen so asto produce silicon carbide powder relatively free of oxygen.

The carbon source reactant should also be readily volatile at thetemperatures in the reaction zone and capable of reacting with thesilane and boron source reactants in a thermodynamically favorablemanner at the most favorable reaction temperature for the production ofsilicon carbide. Volatile hydrocarbons, halogenated hydrocarbons, ormixtures thereof that are substantially pure and substantially free ofchemically combined oxygen can be used as the carbon source reactant. Asused herein, the term"halogenated hydrocarbon", e.g., "chlorinatedhydrocarbon", is intended to mean and include both the compounds ofcarbon, halogen, and hydrogen, and compounds only of carbon and halogen,e.g., carbon tetrachloride.

Typical of hydrocarbons that can be used as the carbon source reactantinclude the normally gaseous or liquid but relatively volatilehydrocarbons, including saturated and unsaturated C₁ -C₁₂ hydrocarbons,such as methane, ethane, propane, the butanes, the pentanes, decanes,dodecanes, ethylene, propylene, the butylenes, and amylenes, symmetricaldimethylethylene and like alkenes; cycloaliphatic and aromatichydrocarbons, such as cyclopentane, cyclohexene, cyclohexane, toluene,benzene, etc.; and acetylenic compounds of which may be noted acetylene,methyl acetylene, ethyl acetylene, and dimethyl acetylene. Rarely arehydrocarbons of more than 12 carbons used.

Examples of halohydrocarbons and halocarbon compounds that can be usedas the source of carbon in the process described herein includesaturated and unsaturated compounds containing from 1 to 12, moreusually 1 to 8, carbon atoms. Examples include: methyl chloride, ethylchloride, chloroform, carbon tetrachloride, dichlorodifluoromethane,n-propyl chloride, amyl chloride, vinyl chloride, 1,1-dichloroethylene,cis and trans 1,2-dichloroethylene, 1,1-dichloroethane,1,2-dichloroethane, ethylene dibromide, trichloroethylene,perchloroethylene, propylene dichloride, 1,1,2-trichloroethane,1,1,1-trichloroethane, 1,1,1,2- and 1,1,2,2-tetrachloroethane,hexachloroethane, and like aliphatic chlorides, fluorides, bromides, oriodides containing up to about 12 carbon atoms, most preferably up toabout 6 carbon atoms. Aromatic halocarbon compounds, e.g., chlorocarboncompounds, also can be used. Such compounds include C₆ -C₉ halogenatedaromatic compounds such as monochlorobenzene, orthodiochlorobenzene,paradichlorobenzene and the like. Cycloaliphatic halides such as the C₅-C₆ aliphatic halides, e.g., chlorinated cyclopentadiene,cyclohexylchloride, etc. can also be used. Preferably, the halogen ofthe halogenated hydrocarbon is the same as the halide of the silane andboron source reactants introduced into the reactor.

Typically, the above-described hydrocarbons and halogenated hydrocarbonsshould be readily vaporizable (volatile) without tar formation sinceotherwise unnecessary difficulties which are unrelated to the processitself can arise, such as plugging of lines by decomposition and/orpolymerization products produced in the course of vaporizing thesereactant materials. Use of hydrocarbons, solely as the carbon sourcereactant generally results in the production of very fine submicronsilicon carbide and deposits of silicon carbide on exposed portions ofthe reactor apparatus adjacent to the reaction zone. Consequently, useof a hydrocarbon source reactant as the sole carbon source reactant isnot preferred. Preferably, halogenated hydrocarbons or mixtures ofhydrocarbons and halogenated hydrocarbons are used.

The amount of carbon source reactant used will preferably be in at leaststoichiometric quantities, i.e., in an amount sufficient to provide atleast 1 atom of carbon for each atom of silicon and 1 atom of carbon foreach 4 atoms of boron introduced into the reaction zone of the reactoras the silane and boron source reactants. The atomic ratio of the carbonsource reactant to the silane and boron source reactants can, of course,vary from stoichiometric quantities. Preferably, greater than thestoichiometric ratio is used for the reason that when a stoichiometricexcess of the carbon source reactant is used, less residual unreactedsilane and boron source reactants are found in the reactor effluent. Theamount of carbon source reactant used in excess of stoichiometric shouldnot be such as to produce large amounts of free carbon in theboron-containing silicon carbide powder product for the reason that toomuch free carbon in the submicron silicon carbide powder product can bedetrimental and its removal requires extra processing of the product.When silicon carbide powder containing large amounts of free carbon isformed into shapes and sintered, the free carbon can appear asobjectionable inclusions of carbon in the microstructure.

Typically, the amount of carbon source reactant should be controlled toobtain between about 0.05 and about 1.5 weight percent, e.g., between0.1 and 1.0 weight percent, free carbon based on the silicon carbidepowder product produced. Free carbon is the carbon present in thesilicon carbide product as elemental carbon and not as chemicallycombined carbon, i.e., as silicon carbide (SiC) or boron carbides, e.g.,B₄ C. Free carbon in silicon carbide powder is reported to act as ascavenger for both oxygen and elemental silicon that may be present inthe powder product or formed during sintering. As a scavenger foroxygen, i.e., a deoxidizer, the free carbon can react with oxygen(elemental or chemically combined, e.g., as SiO, SiO₂ or B₂ O₃) at thesintering temperatures to produce carbon monoxide or carbon dioxide,thereby removing oxygen from the sintered article. Further, the freecarbon can react with elemental silicon or boron under such conditions,as is well understood in the art to form, further silicon carbide orboron carbide. Both oxygen and elemental silicon are reported to hinderdensification and consequently their removal may enhance densificationto high densities. In particular, it has been proposed that an oxidelayer, e.g., a silicon oxide layer, around silicon carbide particlesforms a diffusion barrier which inhibits grain growth. Removal of theoxide layer by free carbon promotes sintering and controls grain growth.

Boron-containing silicon carbide powder prepared by the processdescribed herein is a submicron powder that is comprised substantiallyof isometric crystals of beta-phase (as confirmed by X-ray diffractionanalysis) silicon carbide which are bounded by a complex of crystalfacets or irregular growth forms producing rounded but rough-surfacedparticles. The particles are equant to sub-spherical in habit. Thepowder product is freely dispersible by virtue of limited particleintergrowth. Traces, perhaps up to one weight percent, of alphapolytypes of silicon carbide do occur with the beta phase. Betterresolution of the amount of alpha polytypes is constrained by thesensitivity of the analytical technique used.

Submicron beta silicon carbide powders prepared in accordance with theprocess described are substantially free of undesirable metalcontaminants, i.e., the powder is essentially pure, as established byemission spectrographic analysis.

Metal impurities (analyzed as elemental metal) normally represent lessthan 1000 parts per million parts of the silicon carbide powder (ppm),e.g., less than 0.1 weight percent, and often represent less than 800ppm (0.08 weight percent). Among the metals that can comprise theaforementioned impurities are the following: iron, aluminum, calcium,magnesium, titanium, manganese, tungsten, copper, molybdenum, andvanadium. The source of such metal impurities, if present, in thesilicon carbide powder product is normally the reactants or equipmentused to prepare the product. Since boron is intentionally added, it isnot considered an impurity.

Oxygen and halogen, e.g., chlorine, normally make up the largestindividual non-metallic impurities that are introduced into the productfrom the reactants. By virtue of the process described herein, it isreadily feasible to obtain silicon carbide powder with less than 0.20weight percent halogen, e.g., chlorine, and less than 0.50, e.g., 0.40,weight percent oxygen. By careful recovery, e.g., degasification,techniques, silicon carbide powder with less than 0.15 often less than1.10 weight percent halogen, e.g., chlorine; and less than 0.20 weightpercent oxygen can be obtained. The aforementioned values for halogenand oxygen are based upon analysis for such impurities obtained by useof X-ray spectrographic analysis and by the use of a Leco oxygenanalyzer respectively. The aforementioned X-ray spectrographic techniqueanalyzes principally for unreacted metal halides and subhalides presentin the silicon carbide powder. Adsorbed hydrogen halide, e.g., hydrogenchloride, on the silicon carbide surface may not be detected by thattechnique.

Thus, despite the use of substantially pure reactants and carefulhandling and recovery techniques, a small amount of metal impurities,halogen and oxygen can be present in the silicon carbide powder product.Since boron and free carbon are intentionally present, such elements arenot considered as impurities for purposes of this discussion. The totalamount of the aforesaid impurities is usually less than about 1 weightpercent. Stated another way, the silicon carbide powder is usually atleast about 99 percent pure.

The boron-containing, beta silicon carbide powder produced by theprocess described herein is predominantly submicron in size. The surfacearea of the silicon carbide powder product produced thereby commonlyvaries between about 3 and about 15 square meters per gram (m² /gram),more typically between about 4 and about 12 (m² /gram), e.g., between 5and 10 (m² /gram), as measured by the method of Brunauer, Emmett, andTeller, J. Am. Chem. Soc., 60, 309 (1938). This method, which is oftenreferred as the B.E.T. method, measures the absolute surface area of amaterial by measuring the amount of gas adsorbed under specialconditions of low temperature and pressure. The B.E.T. surface areas asreported herein were obtained using nitrogen as the gas adsorbed andliquid nitrogen temperatures (-196° C.) and a pressure of 150 mm. ofmercury (0.2 relative pressure). For particular cold forming techniques,e.g., injection molding, surface areas of between about 4 and 8 m² /gramare preferred.

Substantially all, i.e., at least 90 percent by number of the siliconcarbide particles comprising the silicon carbide powder are submicron,i.e., have an equivalent spherical diameter of less than one micron. Theequivalent spherical diameter is the diameter of a sphere of equivalentvolume formed by a Zeiss TGZ-3 Particle Size Analyzer and matched to theparticle viewed under high magnification, e.g., 25,000 magnification, asviewed by an electron microscope and depicted in electron micrographs.The preponderent number, i.e., greater than 50 percent, of the particlesless than 1 micron in size are in the particle size range between 0.05and 0.7 microns. Particles as small as 0.03 microns and as large as 5microns can be present in the powdery product; but, particles greaterthan 5 microns rarely represent more than 1 percent by number of theproduct. The aforesaid particles, less than 0.05 microns in size aredistinguishable from ultrafine fragments less than 0.05 microns in sizefound in silicon carbide powder that has been milled extensively. Thesilicon carbide powder described herein is substantially free offragments less than 0.1 micron, e.g., the ultrafine fragments less than0.05 micron.

It is estimated from a study of the silicon carbide powders of thepresent invention with a Zeiss TGZ-3 particle size analyzer that atleast 60 percent on a number basis, more usually at least 70 percent,e.g., 98 percent of the silicon carbide particles comprising the powderare 0.7 microns or less. It is not uncommon to find that the aforesaidpercentages represent also the particles within the particle size rangebetween 0.05 and 0.7 microns. It is estimated further that less than 15percent on a number basis of the silicon carbide particles are greaterthan 1 micron. The aforementioned values respecting the percentage ofcarbide particles 0.7 microns or less depends on the particle sizedistribution of the powder. Generally, the particle size distribution isrelatively narrow. The number mean particle size of the silicon carbideparticles comprising the silicon carbide powder composition is usuallybetween 0.08 and 0.8 microns, more usually between 0.15 and 0.4 micronsand varies directly with the surface area of the powder.

Boron-containing, submicron beta silicon carbide powder of the presentinvention is prepared conveniently by vapor phase reaction ofvaporizable silane compound, e.g., silicon tetrachloride, carbon sourcecompound, e.g., halogenated hydrocarbon, and boron souce compound, e.g.,boron trichloride, in the presence of hydrogen using the equipmentdescribed in U.S. Pat. No. 3,761,576 and particularly FIG. 3 of thatpatent. Briefly, the equipment described in the aforementioned U.S.patent comprises plasma generator heating means mounted coaxially atop areactant inlet assembly which, in turn, is mounted coaxially atop areactor vessel. The solid reaction product is separated from the gaseousproduct effluent in cyclones and the solid product recovered inreceivers connected to the cyclone separators.

In the operation of the aforesaid equipment, plasma gas, e.g., hydrogen,is passed through and heated by the plasma generator heating means. Thehighly heated plasma gas, e.g., a hot hydrogen plasma stream, isdischarged from the plasma generator heating means as a highly heatedgas stream and passes through the reactant inlet assembly. Thereactants, e.g., silicon tetrachloride, boron trichloride and vinylchloride are introduced into the highly heated gas stream fromhorizontal conduits vertically disposed within the inlet assembly andperpendicular to the heated gas stream. The reactants merge with thehighly heated gas (hydrogen) stream and are forwarded into the reactionzone within the reactor where the formation of boron-containing,submicron beta silicon carbide occurs. The reactor effluent whichcomprises a gaseous suspension of solid silicon carbide particles, iswithdrawn from the reactor and separated into its gaseous and solidportions, e.g., by cyclones, filters, etc., and the solid productrecovered.

While the plasma generator heating means shown in FIG. 3 of U.S. Pat.No. 3,761,576 is a direct current arc heater, other plasma heater types,e.g., an induction (high frequency) heater, alternating current archeater, or electrical resistance heater can also be used. The plasma gasis heated typically to a temperature which is sufficient to establishand maintain beta silicon carbide-forming temperatures in the reactionzone bearing in mind that the plasma gas is commonly mixed with silane,boron and carbon source reactants which are introduced commonly at belowthe reaction temperature, usually significantly below the reactiontemperature. The heat content of the reactants can be taken into accountin calculating the temperature required for the plasma gas. Typicallythe principal source of heat for the reaction is the plasma gas. In theproduction of silicon carbide by the aforementioned process and usinghydrogen as the plasma gas, reaction temperatures in the principalreaction zone are calculated to be in the range of between about 2500°C. and about 3500° C.

Typically, hydrogen is used as the gas which is heated by theaforementioned plasma generator heating means, i.e., the plasma gas;while other gases, e.g., the noble gases such as argon and helium, canbe used, especially useful silicon carbide powders have been prepared byuse of a hydrogen plasma. A hydrogen plasma can have an enthalpy ofbetween about 20,000 and 60,000 BTU's per pound of gas, more commonlybetween about 30,000 and 40,000 BUT's per pound. The use of hydrogen asa plasma gas is advantageous since it ensures the establishment of areducing atmosphere and provides a halogen, e.g., chlorine, acceptor inthe reaction zone thereby removing halogen released from the reactants,e.g., the tetrahalosilane, boron halide and, if used, the halogenatedhydrocarbon reactants as hydrogen halide. Mixtures of hydrogen withother gases, such as the aforesaid noble gases, can also be used as theplasma gas. When a noble gas is used as the plasma gas, the hydrogenrequired to establish the aforementioned reducing atmosphere for thevapor phase reaction is introduced into the reactor with one or more ofthe reactants as a carrier gas, as a part of the reactants chemicalstructure, e.g., hydroborides, hydrosilane or hydrocarbon; and/or as aseparate gas stream.

The amount of hydrogen utilized in the above-described process should beat least that amount which is required stoichiometrically to satisfy thetheoretical demand as a halogen acceptor in the reaction. Thetheoretical demand of hydrogen is the amount necessary to combine withall of the halogen introduced into the reactor by the reactants toproduce hydrogen halide, taking into account hydrogen available fromother sources, e.g., the reactants, present in the reactor. Typically,the amount of hydrogen used is in excess of the theoretical amount.Often, the amount of hydrogen utilized will be from 2 to 10 times ormore, the amount of theoretical hydrogen required for the reaction beingconducted. Typically, the mole ratio of hydrogen to silane reactant,e.g., the tetrahalosilane, will range between about 20 and 40, e.g., 25,moles of hydrogen per mole of tetrahalosilane reactant.

The silicon, boron and carbon source reactants which are projected intothe periphery of the highly heated plasma gas stream, e.g., the hothydrogen stream, passing through the reactant inlet assembly, can bethus projected as a combined single stream or as two or more separatestreams. If the reactants are combined for introduction into thereactor, they should be maintained well below rection temperaturespreceding their introduction to prevent premature reaction. Preferably,each of the reactangs is introduced separately through its individualinlet conduit within the reactant inlet assembly.

The reactor shown in U.S. Pat. No. 3,761,576 is a recirculating typereactor as opposed to a plug flow type reactor. The apparent residencetime of the reactants introduced into the reactor is between about 0.05and about 0.5 seconds, more usually between 0.1 and 0.2 seconds. Theapparent residence time is calculated by dividing the reactor volume bythe gas flow through the reactor. The reactor and cyclone separators aretypically cooled externally to provide cooling of the reactor andproduct effluent.

The silicon carbide powder product prepared in accordance with theaforementioned described process is a finely-divided powder that canadsorb gases, such as unreacted reactants that may be present in thereceiver in which the silicon carbide is collected. To avoidcontamination by adsorption, the receivers can be heated to temperaturesabove about 200° F. (93° C.), e.g., from 200° F.-600° F. (93° C.-316°C.) to assist in degassing of the product during collection.Simultaneously, it is advantageous to maintain a stream of hydrogen orchemically inert gas, e.g., a noble gas such as argon, percolatingthrough the product to further assist in the degasification step. In theevent the silicon carbide powder product contains adsorbedchlorine-containing species, e.g., halides of the silicon and boronhalide reactants, such reactants can be removed by heating the productto between about 400° C. and 1000° C., e.g., 500° C.-700° C. andpreferably about 600° C., for between about 1 and 4 hours. Followingdegasification, if employed, the silicon carbide powder product isallowed to cool, e.g., to from about 20° C. to about 100° C.

Boron-containing, submicron beta silicon carbide powder produced by theprocess described herein can be hot pressed, or cold pressed andsintered to solid shapes having a density of at least 85 percent oftheoretical. Preferably, the densified articles have densities of atleast 90, e.g., 95 or 98 percent of theoretical. Articles of highdensity e.g., 96 percent of theoretical, prepared from the aforesaidpowder are expected to have the excellent oxidation resistance,hardness, wear resistance, and thermal-shock resistance typical ofself-bonded silicon carbide. Flexural strengths in excess of 100,000pounds per square inch (psi.) have been obtained with sintered rod-likearticles of 98 percent of theoretical density and flexural strengths inexcess of 60,000 psi. with sintered rectangular articles of 94 percentof theoretical density.

A unique feature of the boron-containing, submicron beta silicon carbidepowder described herein is that it can be formed into complex shapedarticles by conventional techniques used in the field of ceramics; and,such shapes can be densified to greater than 90 percent of thetheoretical density of silicon carbide by vacuum sintering inconventional high temperature furnaces. The surprising ability of theaforesaid silicon carbide powder to be densified to high densities bycold pressing and sintering techniques (pressure-less sintering), i.e.,without the simultaneous application of high external pressures and hightemperatures, allows the use of this silicon carbide for the preparationof complex shapes such as blades, vanes, etc., without the expensive hotpressing techniques and machining required typically in the past.Naturally, the silicon carbide powder of this invention can be hotpressed to shaped articles of high density--even approaching theoreticaldensity--by conventional hot pressing techniques well known to thoseskilled in the art.

Any of the conventional techniques used in the field of ceramic formingcan be utilized with the above-described silicon carbide powder. Thesetechniques include mechanical die pressing, isostatic pressing, slipcasting, extrusion and injection molding. Many of the aforementionedtechniques, require the addition of dispersants, lubricants, binders,etc. to the silicon carbide powder. Such additives are well known topersons skilled in the art of ceramic-forming. The shapes produced byconventional techniques have sufficient green strength to allow theshape to be subsequently handled and fired, which may include apre-firing, to eliminate the aforementioned additives in the hightemperature furnace. Typically, the green density of cold formed shapeswill be in excess of 50 percent of the theoretical density.

Sintering of the cold formed shapes to high density can be accomplishedat a peak sintering temperature between about 1850° C. and 2150° C.,e.g., 2100° C. Heating cycles, i.e., the time-temperature scheduleduring which the shape is heated to and maintained at the peak sinteringtemperature are conventional and can be determined easily by thoseskilled in the art by routine experimentation to optimize thetemperature-density relationship. Typically, the sintering time at thepeak sintering temperature is that time interval which is sufficient toobtain a ceramic article having a density of at least 85 percent,preferably at least 90 or 95 percent and most preferably at least 98percent of the theoretical density of silicon carbide. Commonly, theheating cycle time interval, i.e., from the start of heat-up to theconclusion of sintering will range between about 4 and about 8 hours.The time at the peak sintering temperature can vary and will depend onthe peak temperature used. Generally, the higher the peak temperature,the shorter the time. Care should be taken in limiting the amount oftime at peak temperature so as not to induce the beta to alpha crystaltransformation and resulting coarsening of the microstructure.

The atmosphere in which shaped articles made of the silicon carbidepowder are sintered should be non-oxidizing, i.e., insert, to thesilicon carbide shaped articles. Suitable atmospheres include argon,helium, nitrogen, hydrogen, carbon monoxide, mixtures thereof andsubatmospheric conditions, i.e., a vacuum. Vacuum pressures of less than50 micrometers of mercury, e.g., less than 30 micrometers of mercury arecommonly employed. It has been reported that nitrogen suppresses orretards the beta to alpha crystal transformation of beta siliconcarbide. Consequently, a nitrogen sintering atmosphere may bebeneficial. The boron-containing, submicron beta silicon carbide powderof the present invention has been sintered in a vacuum furnace at a peaksintering temperature of 2034° C. to a high density while retaining thebeta crystalline form as the dominant crystal form, i.e., less than 20percent of the crystals were of the alpha polytypes. Sintered articlesin which the preponderant crystal form is of the alpha polytypes canalso be produced. It is reported that, in addition to the sinteringatmosphere, the beta to alpha crystal transformation is a function ofthe temperature and pressure of sintering. See, for example, "TheConversion of Cubic and Hexagonal Silicon Carbide as a Function ofTemperature and Pressure", by C. E. Ryan et al., AFCRL-67-0436 (1967),Physical Sciences Research Papers No. 336, pp. 177-197.

The cubic beta phase silicon carbide of which the powders are composedcan be largely retained in sintered bodies fabricated from thosepowders. Such sintered bodies, therefore, may consist of fine equantgrains of the beta phase silicon carbide with the coarser elongate platycrystals of alpha phase silicon carbide limited to a content estimatedat less than 20 percent. The apparent grain size, i.e., average diameterof the refractory silicon carbide grain as measured on a polished andetched surface of a sintered specimen is extremly fine. As measured onphotomicrographs of the polished surface, the grain size of the carbidegrains is generally less than ten microns and predominantly in the rangeof about 0.5 to 5 microns. The grains are of relatively uniform size andoccur in a microstructure characterized by contiguous grain boundariesand low porosity resulting in high density and strength of the sinteredbodies.

The silicon carbide powder described herein can be shaped byconventional ceramic forming techniques and sintered to a high densityto provide a variety of articles having high ressitance to thermalstress and shock, corrosion resistance to high temperature oxidizingatmospheres and good wear resistance. Such shapes have applications inthe field of engineering, ceramic components operating at hightemperature and oxidizing atmospheres with varying degrees of stress andsuch components operating in abrasive environments. Major applicationsfor high temperature use include rocket nozzles, resistance heaters,radiant heater tubes, gas turbine components, e.g., air foils andblades, reciprocating engines, and chemical plant components. Forwear-resistance applications, there can be mentioned mechanical seals,bearings and spinnerets. Articles prepared from the submicron siliconcarbide powder described herein are particularly applicable for use ascomponents for gas turbines employed for electric power generation andfor automotive gas turbine engines.

The present invention is more particularly described in the followingexamples which are intended as illustrative only since numerousmodifications and variations therein will be apparent to those skilledin the art. In the following examples, volumes of hydrogen gas areexpressed in cubic feet per hour at standard conditions (14.7 pounds persquare inch and 21° C.) or SCFH. Reactant gas stream rates were measuredat nominal laboratory conditions, i.e., one atmosphere and 21° C. andare reported as measured if other than SCFH. Unless otherwise specified,all percentages are by weight. Specific reaction conditions recited inthe Examples are representative of the operating conditions since flowrates tend to vary with time.

The plasma generator used in the Examples was a medium voltage, mediumamperage direct current arc heater having a power input of 28 kilowatts.The arc heater was operated at between 24 and 28 kilowatts and with anefficiency of from about 50 to 75 percent. Hydrogen was used as theplasma gas. The plasma heater was mounted coaxially and vertically atopa reactant inlet mixer assembly having three horizontal reactant inletconduits in vertical alignment as shown in FIG. 3 of U.S. Pat. No.3,761,576. The heated hydrogen gas was discharged from the arc heaterthrough the mixer assembly and thence into the reactor. A field coil waspositioned around the anode to assist in stabilizing the arc when plasmagas was introduced radially into the region between the anode and thecathode of the plasma heater. The mixer assembly was mounted coaxiallyatop a reactor vessel.

EXAMPLE I

A direct current plasma arc heater, as described hereinabove, wasconnected to a source of direct current and an arc struck between apointed stick cathode and hollow cylindrical anode. The arc heater wasoperated at about 149 volts and 168 amperes, i.e., about 25.0 kilowatts.Three hundred SCFH of hydrogen plasma gas was introdued radially intothe region between the cathode and anode and heated by the arc as itpassed through the anode. The field coil around the anode was operatedat 48 amps and 21 volts. The hot hydrogen gas stream discharging fromthe nozzle of the cylindrical anode was passed through the reactantinlet assembly and thence into the reactor vessel.

Vinyl chloride and 45 SCFH hydrogen carrier gas were introduced into thehot hydrogen stream from the top reactant inlet conduit of the reactantinlet mixer assembly. The vinyl chloride was used in an amountcalculated to be 25 percent in excess of stoichiometry. Seventy SCFH ofhydrogen were introduced into the hot hydrogen stream through the middleconduit of the mixer assembly. 102 millimoles per minute of silicontetrachloride, 76.2 cc/minute of boron trichloride and 20 SCFH ofhydrogen carrier gas were introduced into the hot hydrogen streamthrough the bottom conduit of the mixer assembly. This represented anominal rate of 0.5 pound of silicon carbide per hour. During the totalperiod of operation, the rate of production was varied slightly from the0.5 pound per hour rate and accordingly other reaction conditions alsovaried slightly; however, the principal rate of production was at the0.5 pound per hour rate. Arc heater efficiency was calculated at onepoint in the operation of the equipment to be about 61 percent. Analysesof the product indicated that the level of boron in the silicon carbideproduct was between 0.34 and about 0.4 weight percent. The siliconcarbide product was found to be essentially of the beta crystallineform, have a B.E.T. surface area of about 14 square meters per gram,have a total carbon content of about 30.6 percent and a chlorine contentof about 0.06 percent.

EXAMPLE II

A portion of the silicon carbide powder of Example I was isostaticallypressed in a 3/4 inch diameter×3 inch length rubber mold at 20,000pounds per square inch. The compacted rod was approximately 1/2 inch indiameter by 2 inches in length. The compacted rod was sintered in avacuum furnace at a peak temperature of about 2110° C. Heating time fromroom temperature to 2,000° C. was 10 hours. The time at temperatures of2,000° C. and higher was about 45 minutes. The sample was permitted tocool overnight in the furnace and when removed was found to have adensity of 3.04 grams/cc or 94.5 percent of the theoretical density ofsilicon carbide.

EXAMPLE III

Submicron beta silicon carbide powder was prepared following theprocedure of Example I except that the rate of silicon tetrachloride was85 millimole per minute and the rate of boron trichloride was 191cc/minute. The arc heater power level was 23.5 kilowatts and at onepoint during the period of operation had a measured efficiency of 61.5percent. The silicon carbide powder was found to contain from about 1.6to 1.7 weight percent boron, have a B.E.T. surface area of about 11square meters per gram, a total carbon content of 30.3 percent, and achlorine content of about 0.07-0.08 percent.

A portion of the silicon carbide powder was used to prepare a sinteredrod using the procedure of Example II. Test specimens cut from differentends of the rod were found to have densities of 94 and 84 percent oftheoretical. The rod was found to have been poorly packed at the endhaving the relatively low density.

EXAMPLE IV

Submicron beta silicon carbide powder was prepared following theprocedure of Example I except that the rate of silicon tetrachloride was85 millimoles per minute and the rate of boron trichloride was 145cc/minute. At various periods during the production run, the rate ofsilicon tetrachloride was increased to 102 millimoles per minute and therate of boron trichloride was increased to 174 cc/minute. Arc heaterefficiency varied from about 58 to 61 percent; power levels varied from22 to 25 kilowatts. The silicon carbide powder was found to containabout 0.94 percent boron, have a B.E.T. surface area of 12 square metersper gram, a total carbon content of 30.4 percent and a chlorine contentof about 0.07 percent.

A portion of the silicon carbide powder was used to prepare a sinteredrod using the procedure of Example II. The rod was found to have adensity of about 93.5 percent of theoretical.

EXAMPLE V

Submicron beta silicon carbide powder was prepared following theprocedure of Example I except that the arc heater power level was about30 kilowatts; the hydrogen rate through the top, middle and bottomconduits of the mixer assembly were 85.5, 85.5 and 30 SCFH respectively;the production rate was between 1 and 1.5 pounds of silicon carbide perhour; the vinyl chloride rate was used in an amount between 5 and 15percent excess; and the boron trichloride rate was 150 cc/minute and 230cc/minute at the 1 and 1.5 pound/hour rates. The silicon carbide powderproduct contained about 0.27 percent boron, as measured by emissionspectrographic analysis; had a B.E.T. surface area of 6.6 m² /gram; atotal carbon content of 30.6 percent; a free carbon content of 0.85percent; an oxygen content of 0.17 percent; and a chlorine content of0.02 percent. The powder was slurried in 1,1,1-trichloroethane solvent,screened through a 400 mesh screen and vacuum dried to remove thesolvent. A portion of the powder was isostatically pressed at 20,000lbs. per square inch in a rubber mold. The resulting green body wassintered in a vacuum furnace for 1 hour at 2100° C. Time to achieve thetemperature of 2100° C. was 51/2 hours. After being allowed to cool toroom temperature, the density of the sintered shape was determined andfound to be 98.5 percent of theoretical.

EXAMPLE VI

Submicron beta silicon carbide powder was prepared following theprocedure of Example I except that the boron trichloride reactant wasomitted; the arc heater power level was about 35 kilowatts; arc heaterefficiency varied from 49-60 percent; the field coil was operated at 77amps; and ethyl chloride was used as the carbon source reactant. Silicontetrachloride was introduced into the hot hydrogen stream at rates of490 and 582 millimoles per minute. The corresponding rates forethylchloride were 6.1 and 7.3 liters per minute. The rate of hydrogenintroduction through the top, middle and bottom conduits were 30, 128and 60 SCFH respectively.

The submicron beta silicon carbide produced had a surface area of about6.4 square meters per gram; contained less than 0.01 weight percentboron, a total carbon content of 30.2 percent, a free carbon content of0.73 percent, a chlorine content of 0.02 percent and an oxygen contentof 0.11 percent. A portion of this powder was isostatically pressed andsintered in the manner described in Example V. The sintered shape had adensity of 57.2 percent of theoretical.

EXAMPLE VII

The submicron beta silicon carbide powders of Examples V and VI wereblended in various proportions so that the calculated boronconcentration of the blend varied from 0.15 to 0.24 weight percent. Thepowders were blended for one hour in a polyethylene bottle with tungstencarbide cycloids in order to obtain a homogeneous blend. A portion ofeach of the blends was isostatically pressed and sintered in a vacuumfurnace in the manner described in Example V. Results are tabulated inTable I.

                  TABLE I                                                         ______________________________________                                        Blend   % Boron      % Theoretical Density                                    ______________________________________                                        A       0.15         83.4                                                       B*    0.17         91.6                                                     C       0.20         96.1                                                     D       0.24         98.0                                                     ______________________________________                                         *Blended with polyethylene balls.                                        

The data of Table I show that as little as about 0.15 percent ofconformed boron in submicron beta silicon carbide provides a productwhich can be cold pressed and sintered to densities of about 85 percentof theoretical; and, as little as 0.17 percent of coformed boron issufficient to obtain a sintered article of at least 90 percent oftheoretical density. Further, when the level of coformed boron is in therange of 0.20-24 percent, densities in excess of 95, e.g., 96 and 98percent of theoretical, were obtained.

The data show also that essentially boron-free submicron beta siliconcarbide powder can be sintered to high densities by blending with suchpowder submicron beta silicon carbide containing coformed boron. It iscontemplated that from 70 to 10 weight percent, e.g., 55 to 15 weightpercent, of substantially boron-free submicron silicon carbide can beblended with from 30 to 90 weight percent, e.g., 45 to 85 weightpercent, of submicron silicon carbide containing coformed boron. Theamount of coformed boron can vary; but, will be in an amount such thatthe blend contains boron within the range of 0.15 to 4 weight percent,as described hereinbefore.

EXAMPLE VIII (Comparative Examples)

A series of silicon carbide powders were prepared as detailedhereinafter. Boron carbide (B₄ C) having a surface area of 10.3 m² /gramwas used to prepare boron-containing powders. Each of the powder blendswere formulated to have a total carbon content of between 30.4 and 31.6weight percent and except for Powder F had the equivalent of about 1weight percent boron carbide.

Powder A

A commercial silicon carbide powder was obtained and classified bysettling and decantation in water. The silicon carbide was of the alphacrystalline forms and the classified portion had a B.E.T. surface areaof 8.2 square meters per gram. To the sample was added one weightpercent of the boron carbide and sufficient carbon black to make thetotal carbon content of the sample equal 30.4 percent. The boron carbideand carbon were blended with the silicon carbide. The paraffin wax wasadded to the blend dissolved in 1,1,1-trichloroethane solvent. After thewax was distributed well on the powder blend, the solvent wasevaporated.

Powder B

Submicron silicon carbide powder was prepared by feeding silicontetrachloride and methane to a hydrogen plasma in accordance with theprocess described in U.S. Pat. No. 3,340,020. The B.E.T. surface area ofthe silicon carbide powder was 51 square meters/gram. To this powder wasadded one weight percent of boron carbide and sufficient carbon black tomake the total carbon content of the sample equal 30.4 percent. Theboron carbide, carbon and silicon carbide were blended following theprocedure described with respect to the Powder A.

Powder C

Submicron beta silicon carbide powder was prepared in accordance withthe process described in U.S. Pat. No. 3,271,109. The process describedin the aforementioned patent comprises reacting finely-divided silicaand carbon black in a flowing inert gaseous atmosphere at temperaturesin excess of 1150° C. The silicon carbide powder had a B.E.T. surfacearea of 1.8 square meters/gram and a total carbon content of 31.6percent. To the sample was added one weight percent of boron carbide. Noadditional carbon was added. The paraffin wax added following theprocedure described with respect to Powder A.

Powder D

Submicron beta silicon carbide powder was prepared following theprocedure of Example I except that the arc heater power level was about35 kilowatts; the arc heater efficiency was about 48 percent; the fieldcoil was operated at 55 amps; the vinyl chloride, silicon tetrachlorideand boron trichloride rates were 5.3 liters/minute, 390millimoles/minute and 475 cc/minute respectively; and the hydrogen ratesthrough the top, middle and bottom conduits were 120, 171 and 30 SCFHrespectively. The aforementioned conditions were representative of theoperating conditions used to produce the silicon carbide powder. Thesilicon carbide powder had a surface area of 8.0 square meters/gram anda total carbon content of 30.7 percent. The boron content was 0.58percent. To this sample was added 0.5 weight percent of boron carbide.No additional carbon was added. The paraffin wax was added following theprocedure described with respect to Powder A.

Powder E

Submicron beta silicon carbide powder was produced following theprocedure of Example VI. The silicon carbide powder had a surface areaof 10.1 square meters per gram and a total carbon content of 30.7percent. To this sample was added one weight percent boron carbide. Noadditional carbon was added. The paraffin wax was added following theprocedure described with respect to Powder A.

Powder F

A portion of Powder E was used with no addition of boron carbide.

Each of the aforementioned powders was blended with about 2 weightpercent of paraffin wax (unless otherwise noted) as described and coldpressed at 15,000 pounds per square inch in a mechanical press to asufficient green strength to be handled. Unless otherwise indicated, thearticles were sintered in a vacuum furnace at about 2,000° C. forone-half hour. Heat up time to 2,000° C. was about 41/2 hours. Aftercooling, the densities of the sintered shapes were determined. Thedensities appear in Table II.

                  TABLE II                                                        ______________________________________                                                % Theoretical Density                                                 Powder  Average Values  Comment                                               ______________________________________                                        A       71                                                                    B       83                                                                    C       55.5            5% wax added,                                                                 Peak Temp. = 1960° C.                          D       91                                                                    E       93.7            Peak Temp. = 1960° C.                          F       55.8            Peak Temp. = 1965° C.                          ______________________________________                                    

The data of Table II show that only submicron silicon carbide preparedby the process described herein and formulated to contain boron aredensified to greater than 90 percent of theoretical by cold forming andsintering techniques.

Example IX

Submicron beta silicon carbide powder was prepared following theprocedure of Example I except: the arc heater power was 35 kilowatts,the arc heater efficiency was in the range of 56-58 percent, the fieldcoil was operated at 77 amps, and ethyl chloride was used as the carbonsource. The rate of ethyl chloride, silicon tetrachloride and borontrichloride were 4.9 liters/minute, 350 millimoles/minute and 475cc/minute respectively. The hydrogen flow rates through the top, middleand bottom conduits of the mixer assembly were 60, 200 and 15 SCFHrespectively.

Boron-containing, submicron beta silicon carbide powder prepared asdescribed was washed with water and screened through a 400 mesh screen.The powder had a B.E.T. surface area of 8.7 square meters/gram, a totalcarbon content of 30.4 percent, a free carbon content of 0.93 percent,an oxygen content of 0.28 percent, and a boron content of 0.36 percent.

A portion of the powder was hot pressed into a disc shape at 2,000° C.and 3,000 pounds per square inch. The density of the disc was found tobe 97.3 percent of theoretical.

A portion of this powder was blended with 3 weight percent of paraffinwax to improve green strength and cold pressed with a mechanical press(5 tons pressure) into a 2 inch×2 inch×174 inch plate. The plate wassintered at 2,100° C. for 1 hour. Time to reach 2,100° C. was 4 hours.The sintered plate had a density of 97.4 percent of theoretical.

The data of Example IX show that the boron-containing, submicron betasilicon carbide of the present invention can be cold pressed andsintered or hot pressed to high densities, e.g., 97 percent oftheoretical.

EXAMPLE X

Submicron beta silicon carbide powder was prepared following theprocedure described with respect to Powder D of Example VIII. Thesurface area of the powder was 7.2 square meters/gram, the total carboncontent was 30.6 percent, the free carbon content was 1.4 percent, theoxygen content was 0.27 percent, and the boron content was 0.52 percent.The powder was slurried in 1,1,1-trichloroethane, screened through a 400mesh screen and dried.

A portion of the powder was blended in a Sigma mixer with paraffin wax.The blend, which was 55 percent by volume silicon carbide powder and 45percent by volume wax, was injection molded into a small ring. The ringwas sintered at 2,000° C. for 10 minutes. Time to reach 2,000° C. was51/2 hours. The sintered ring was found to have a density of 97 percent.

A portion of the powder-wax blend was injection molded into a smallturbine blade about 1 inch high and sintered at 1932° C. Time to reach1932° C. was 8 hours. The blade was found to have a density of 90percent of theoretical.

The data of this Example show that both simple and complex shapes can beproduced using conventional ceramic cold forming techniques, e.g.,injection molding, with the silicon carbide powder of this invention,and the green shapes thus formed sintered to a high density, e.g., 90percent of theoretical.

Although the present process has been described with reference tospecific details of certain embodiments thereof, it is not intended thatsuch details should be regarded as limitations upon the scope of theinvention except as and to the extent that they are included in theaccompanying claims.

I claim:
 1. A substantially pure submicron boron-containing beta siliconcarbide powder composition consisting essentially of silicon carbidepowder substantially all of the beta crystalline form, said powderhaving a surface area of between about 3 and about 15 square meters pergram, the number mean particle size of the silicon carbide particlescomprising the powder being between 0.08 and 0.8 microns, coformedhomogeneously distributed boron-containing additive in an amountequivalent to between 0.15 and 0.25 weight percent boron, based on thesilicon carbide, and between 0.05 and 1.5 weight percent free carbon,based on the silicon carbide.
 2. The powder composition of claim 1wherein the silicon carbide powder surface area is between about 4 and12 square meters per gram.
 3. The powder composition of claim 1 whereinthe number mean particle size of the silicon carbide particles isbetween 0.15 and 0.4 microns.
 4. The powder composition of claim 1wherein the boron-containing additive is boron carbide, elemental boronor mixtures of boron carbide and elemental boron.
 5. The powdercomposition of claim 1 or 4 wherein the silicon carbide powder containsless than 0.1 weight percent metal impurities.
 6. A silicon carbidepowder composition capable of being pressure-less sintered to articleshaving a density of at least 85 percent of the theoretical density ofsilicon carbide, which composition consists essentially of substantiallypure, submicron boron-containing beta silicon carbide powder, saidpowder having from traces to less than 1 weight percent alpha siliconcarbide, and a surface area of between about 3 and about 15 squaremeters per gram, the number mean particle size of the silicon carbideparticles comprising the powder being between 0.08 and 0.8 microns,coformed homogeneously distributed, boron-containing additive in anamount equivalent to between 0.15 and 0.25 weight percent boron, basedon the silicon carbide, and between 0.05 and 1.5 weight percent freecarbon, based on silicon carbide.
 7. The powder composition of claim 6wherein the silicon carbide powder surface area is between about 4 and12 square meters per gram.
 8. The powder composition of claim 6 whereinthe boron-containing additive is boron carbide, elemental boron ormixtures of boron carbide and elemental boron.
 9. The powder compositionof claim 6 or 8 wherein the silicon carbide powder contains less than0.1 weight percent metal impurities.
 10. A silicon carbide powdercomposition consisting essentially of a blend of from about 30 to 90weight percent substantially pure, submicron beta silicon carbidecontaining coformed, homogeneously distributed, boron-containingadditive, and from 70 to 10 weight percent substantially pure,substantially boron-free submicron beta silicon carbide, each of saidbeta silicon carbide powders having from traces to less than 1 weightpercent alpha silicon carbide, and a surface area of between about 3 andabout 15 square meters per gram, the number mean particle size of thesilicon carbide particles comprising the powder being between 0.08 and0.8 microns, said powder blend containing from about 0.15 to about 0.25weight percent boron and between 0.05 and 1.5 weight percent freecarbon, based on silicon carbide.
 11. The silicon carbide powdercomposition of claim 10 wherein the composition consists essentially offrom 45 to 85 weight percent of the boron-containing silicon carbidepowder and from 55 to 15 weight percent of the substantially boron-freesilicon carbide powder and the boron-containing additive is boroncarbide, elemental boron or mixtures of boron carbide and elementalboron.
 12. A substantially pure submicron boron-containing beta siliconcarbide powder composition consisting essentially of silicon carbidepowder substantially all of the beta crystalline form, said powderhaving a surface area of between about 3 and about 15 square meters pergram, the number means particle size of the silicon carbide particlescomprising the powder being between 0.08 and 0.8 microns, coformed,homogeneously distributed, boron-containing additive in an amountequivalent to about 0.27 weight percent boron, based on the siliconcarbide, and between 0.05 and 1.5 weight percent free carbon, based onthe silicon carbide.
 13. A silicon carbide powder composition capable ofbeing pressureless sintered to articles having a density of at least 85percent of the theoretical density of silicon carbide, which compositionconsists essentially of substantially pure, submicron boron-containingbeta silicon carbide powder, said powder having from traces to less than1 weight percent alpha silicon carbide, and a surface area of betweenabout 3 and about 15 square meters per gram, the number mean particlesize of the silicon carbide particles comprising the powder beingbetween 0.08 and 0.8 microns, coformed, homogeneously distributed,boron-containing additive in an amount equivalent to about 0.27 weightpercent boron, based on the silicon carbide, and between 0.05 and 1.5weight percent free carbon, based on silicon carbide.
 14. The powdercomposition of claims 12 or 13 wherein the boron-containing additive isboron carbide, elemental boron or mixtures of boron carbide andelemental boron.
 15. The powder composition of claim 14 wherein thesilicon carbide powder contains less than 0.1 weight percent metalimpurities.
 16. A shaped sintered silicon carbide article consistingessentially of silicon carbide, from about 0.15 to 0.25 weight percentboron and from about 0.05 to about 1.5 weight percent free carbon, saidarticle having a density of at least 85 percent of the theoreticaldensity of silicon carbide.
 17. A shaped sintered silicon carbidearticle consisting essentially of silicon carbide, about 0.27 weightpercent boron and from about 0.05 to about 1.5 weight percent freecarbon, said article having a density of at least 90 percent of thetheoretical density of silicon carbide.
 18. The shaped sintered siliconcarbide article of claims 16 or 17 wherein the beta crystalline form isthe dominant crystal form in the sintered article.
 19. The shapedsintered silicon carbide article of claims 16 or 17 wherein themicrostructure of the article is fine and of relatively uniform size.20. The shaped sintered silicon carbide article of claims 16 or 17wherein the article has a density of at least 95 percent of theoretical.21. A method for preparing a shaped sintered silicon carbide article,which comprises cold pressing into a desired shape a boron-containingsilicon carbide composition consisting essentially of substantiallypure, submicron beta silicon carbide powder, said powder having fromtraces to less than 1 weight percent alpha silicon carbide, a surfacearea of between 3 and about 15 square meters per gram, the number meanparticle size of the silicon carbide particles comprising the powderbeing between 0.08 and 0.8 microns, coformed, homogeneously distributed,boron-containing additive in an amount equivalent to between 0.15 and0.25 weight percent boron, based on silicon carbide, and between 0.05and 1.5 weight percent free carbon, based on silicon carbide, andsintering such article at temperatures of between about 1850° C. and2150° C. in an inert atmosphere until the article has a density of atleast 85 percent of the theoretical density for silicon carbide.
 22. Amethod for preparing a shaped sintered silicon carbide article, whichcomprises cold pressing into a desired shape a boron-containing siliconcarbide composition consisting essentially of substantially pure,submicron beta silicon carbide powder, said powder having from traces toless than 1 weight percent alpha silicon carbide, a surface area ofbetween 3 and about 15 square meters per gram, the number means particlesize of the silicon carbide particles comprising the powder beingbetween 0.08 and 0.8 microns, coformed, homogeneously distributed,boron-containing additive in an amount equivalent to about 0.27 weightpercent boron, based on silicon carbide and between 0.05 and 1.5 weightpercent free carbon, based on silicon carbide, and sintering sucharticle at temperatures of between about 1850° C. and 2150° C. in aninert atmosphere until the article has a density of at least 85 percentof the theoretical density for silicon carbide.
 23. A method accordingto claims 21 or 22 wherein the boron-containing additive is boroncarbide, elemental boron or mixtures of boron carbide and elementalboron.
 24. A method according to claim 23 wherein the silicon carbidepowder contains less than 0.1 weight percent metal impurities.
 25. Amethod according to claim 23 wherein the sintered article has a densityof at least 95 percent of theoretical.